Field of the Invention
This invention relates generally to hybrid assemblies, and more particularly to methods of mounting such hybrid assemblies such that they exhibit good thermal performance.
Description of the Related Art
A hybrid assembly comprises multiple interconnected integrated circuit (IC) dies. A number of methods are known for mounting a hybrid assembly to a baseplate. One possible method is shown in FIG. 1. Here the hybrid assembly 10 includes a first die 12, such as a mercury cadmium telluride (MCT) detector or a gallium antimonide (GaSb) superlattice LED (SLED) array IC, and a second die 14, such as a CMOS readout IC (ROIC) or a read-in IC (RIIC) on a silicon substrate (typically 250 μm thick); the first and second dies would typically be bonded together and electrically interconnected using a layer 16 of indium bumps and epoxy. To mitigate problems that might arise due to the first and second dies having different coefficients of thermal expansion (CTEs), a ‘balanced composite substrate (BCS) stack structure is sometimes used. This requires mounting the second die 14 to a metal layer 18, typically 125 μm thick, using an epoxy adhesive layer 20, and mounting the metal layer 18 to a silicon layer 22, typically 250 μm thick, using an epoxy adhesive layer 24. This structure is then mounted to a baseplate 26, typically using an epoxy adhesive layer 28.
For proper operation, hybrid assemblies such as those described above are often actively cooled. The 250 μm thick silicon layers (14 and 22) are symmetrical about the middle of the stack so that, as the assembly is cooled, the structure does not bend due to different CTEs.
However, this mounting method results in three epoxy joints, each of which is typically about 37 μm thick. Standard epoxy has very poor thermal conductivity (TC) (˜0.1 Watts/(meter*Kelvin)). As such, these epoxy joints dramatically restrict the vertical flow of heat from the CMOS or SLED at the top to the baseplate. Epoxies are available which are filled with high TC powders (silver or boron nitride or diamond) which can increase the TC, but the epoxy layers still disrupt the vertical heat flow. This poor vertical thermal conductivity can limit the performance of hybrid assemblies that inherently generate heat, such as SLEDs, or that operate best at low temperatures, such as IR detectors.